This invention relates to a cassette case for a magnetic tape, and more particularly to a cassette case in which those sections that contact the magnetic tape are roughened.
It is conventional to produce cassette cases for magnetic tapes by injection molding of plastic material. In the past, it has been believed that if a tape-contact section of a magnetic tape cassette has a rough surface, it might, during feeding of the tape, damage the tape at the base surface thereof, leading to a cutting of the tape or influencing the tape winding torque and phase to deteriorate the recordation or reproduction of sound from the tape. Thus, in the past it has been considered necessary that each such tape-contacting section of a cassette case must be finished to have a very smooth surface (a mirror surface).
Heretofore, in producing a metal mold for cassette cases, those portions of the mold corresponding to the tape-contact sections of the cassette case have been carefully worked to provide such a "mirror surface", and hence are subjected to complicated steps to produce such a surface finish, such as machining, grinding, and the like, which significantly raise production costs. Additionally, metal molds must be handled with sufficient care so that the mold portions just described are not damaged.
Recently, it has become practical to form a main part of a cassette case from a die cast product of a metal material for the purpose of further improving the dimensional accuracy of the cassette case, thereby to improve the feeding of the magnetic tape. In such a case, the inner surfaces of the metal mold for die casting are each worked into a mirror surface having a surface roughness of less than about 0.6 micron, so that the corresponding surfaces of the die cast product are each formed into a mirror surface. The tape-contacting sections of the cassette case thus have mirror-like surfaces, and it has been believed that such surfaces of a cassette case are very effective to reduce frictional engagement with the magnetic tape to permit good tape movement.
Providing a mirror surface has its disadvantages. Specifically, to prevent such surfaces from being roughened during a deburring or surface treating operation after the molding of the case, it is necessary to provide masking on the front face of each cassette case. Such masking requires additional operations, increasing the labor to produce the cassette case and raising production costs.
The present invention proceeds from a new understanding of tape movement in a cassette case, in which it has been found that the mirror surface of the tape-contacting section of the case is not always effective for tape movement. It has been found that such tape-contacting sections should be provided with a predetermined surface roughness to enhance tape movement. Specifically, it has been found that where a thin sheet having an extremely smooth surface, such as a magnetic tape, travels on and along a member that presents a mirror surface, both surfaces closely contact each other, thereby increasing friction between them, causing tape sticking and slipping. In this fashion, such mirror surfaces cause the winding or taking-up torque to be increased to ensure regular and uniform feeding of the tape; otherwise, tape movement will deteriorate, causing such problems as wow and flutter, and poor movement of the tape during its initial start-up movement.
As noted above, the present invention proceeds upon the basis of utilizing tape-contacting surfaces with predetermined roughness, rather than with a mirror surface. It has been found that excellent tape movement characteristics can be achieved by providing a tape-contact section of a cassette case with a surface roughened (by honing or the like), in which the surface roughness is within the range of about 3 to 15 microns, and preferably within the range of about 3 to 10 microns.
Accordingly, it is an object of the present invention to eliminate the necessity of careful finishing of a mold corresponding to the tape-contacting section of a cassette case to provide the same with a mirror surface. It is also an object of the present invention to eliminate the necessity of providing masking during deburring or surface treating ooperations as noted above. It is a further object of the invention to simplify the production of cassette cases.
These objects are achieved by providing a tape-contacting section of a cassette case with a surface roughness within the range of about 3 to 15 microns, and preferably within the range of about 3 to 10 microns.
The invention will be more completely understood by reference to the following detailed description.